Recently, various standards have been developed for data communication over broadband wireless links. One such standard is set out in the IEEE 802.16 specifications and is commonly known as WiMAX. Existing specifications include IEEE 802.16-2004, primarily intended for systems having fixed subscriber stations, and IEEE 802.16e-2005 which among other things provides for mobile subscriber stations. Currently under development, the IEEE 802.16m project (also called Advanced WiMAX or Gigabit WiMAX) proposes using advanced techniques, including multiple antennas, to provide high data rates to mobile subscriber stations. In the following description, the term mobile station (MS) is used as shorthand for both mobile and fixed subscriber stations. The term “user” is also used equivalently to mobile station. Further, “legacy MS” or “legacy user” refers to a mobile station operating in accordance with the current WirelessMAN-OFDMA specification (IEEE 802.16e-2005) and using the feature set specified in the WiMAX forum system profile (Release 1.0).
The entire contents of IEEE Std 802.16-2004 “Air Interface for Fixed Broadband Wireless Access Systems” and IEEE Std 802.16e-2005 “Amendment 2 and Corrigendum 1 to IEEE Std 802.16-2004” are hereby incorporated by reference.
In systems of the above type, data is communicated by exchange of packets between the mobile stations and base station whilst a connection (management connection or transport connection), having a connection ID, is maintained between them. The direction of transmission of packets from the subscriber station to the base station is the uplink (UL), and the direction from the base station to the subscriber station is the downlink (DL). Transmission of data packets takes place within “frames” which are the predetermined unit of time in the system, each frame conventionally having one downlink subframe followed by one uplink subframe, these in turn being divided in the time and frequency domain into a number of slots, and when utilising multiple transmit antennas possibly also divided spatially into a number of streams. At the physical layer level, transmission of data involves combining groups of subcarriers (available frequencies in the system) to form “symbols” by employing the well-known technique of OFDMA (Orthogonal Frequency Division Multiple Access). The base station can apply different modulation and coding schemes (MCS) within distinct zones of a subframe, for example to provide high data throughput to nearby users, whilst providing a more robust signal to more distant users or users moving with high mobility.
Various physical layer implementations are possible in an IEEE 802.16 network, depending on the available frequency range and application, including a time division duplex (TDD) mode, which involves operating the two links on the same frequency band, but subdividing the access to the medium in time so that only the DL or the UL will be utilizing the medium at any one point in time. The remainder of this specification will refer to a TDD mode WiMAX system by way of example.
Thus, in the TDD mode, conventionally each frame is divided into one DL subframe followed by one UL subframe, as shown in FIG. 1. The DL-subframe includes a broadcast control field with a DL-MAP and UL-MAP, by which the BS informs the MSs of the allocations within the DL and UL. The MAP is a map of bandwidth allocation in the frame and also contains other PHY signalling related messages. It consists of Information Elements (IE) each containing a connection ID. The map IEs inform mobile stations to which burst(s) they have been assigned to receive or transmit information.
The 802.16e-2005 standard specifies many possible frame durations ranging from 2 ms to 20 ms in length. However, the current WiMAX forum profile (Release 1.0), specifies that only 5 ms frames shall be used as this will ensure that all WiMAX forum certified equipment is interoperable. Although the 5 ms frame is widely accepted, it is believed that use of this frame length will create latency issues which can cause problems for users travelling with medium to high mobility. Users travelling at high mobility may experience rapid changes in channel conditions and will require fast link adaptation in order to sustain adequate performance and throughput. However, with a 5 ms frame this fast link adaptation becomes difficult as the minimum time that any specific MS can have their Modulation and Coding Scheme (MCS) adapted to the prevailing propagation link is 5 ms. In this case, the MS will calculate a channel quality measurement based on the Physical or Effective Carrier-to-Interference-and-Noise Ratio CINR which will provide information on the actual operating condition of the receiver, including interference and noise levels, and signal strength. This information is then fed back to the BS via CQI feedback channel (CQICH) in the uplink and as a result the BS can perform link adaptation for the MS. As mentioned above, users moving at high speeds will experience rapid variations in channel conditions especially within a 5 ms time frame and it is therefore highly possible that an MS will feedback channel quality information that at the time of scheduling will not correctly represent the channel at the time of transmission. The result of this inaccurate representation in channel quality information may degrade the performance and throughput experienced by the MS.
In order to support users travelling at higher speeds then it is apparent that the frame duration must be reduced to facilitate fast and efficient link adaptation. However, as technology evolves then backward compatibility can become a major issue which is particularly true in the case of the current IEEE 802.16m project. The aim of this IEEE project is to provide an amendment to the legacy IEEE 802.16e-2005 standard where the purpose of this amendment is to provide performance improvements necessary to support future advanced services and applications. One requirement of this project is to reduce latency as far as feasible without infringing the strict legacy support requirements. As mentioned above, the frame duration can be reduced which can ultimately reduce latency but this must be achieved so as not to affect the performance of a legacy MS. In other words, the IEEE 802.16m BS must be able support legacy MSs whilst also supporting IEEE 802.16m MSs at a level of performance equivalent to that which a legacy BS provides to a legacy MS.
Referring again to FIG. 1, in the legacy IEEE 802.16e-2005 TDD frame structure, the first symbol is occupied by a Preamble which is mainly used for synchronisation purposes. On the second and third symbols following the Preamble is the frame control header FCH. The FCH is transmitted using a well-known format and provides sufficient information to decode the following MAP message, i.e. the MAP message length, coding scheme and active sub-channels. Following the FCH is the DL-MAP which may be followed by the UL-MAP. These MAP messages provide information on the allocated resource (slots) for traffic channels within the frame. These MAP's contain DL-MAP_IE's and UL-MAP_IE's which define bursts within the frames, (i.e. one MAP_IE will be related to 1 burst within the frame). The information within these MAP_IE's, such as the subchannel offset and symbol offset are crucial as these are used by the MS to locate the resource within the subframes. Other information such as the CID (Connection ID), the modulation and coding scheme and the number of subchannels are also crucial as these will allow for successful demodulation and decoding of the data within the burst. Following the DL and UL MAPs, there may be a Downlink Channel Descriptor (DCD) and/or an Uplink Channel Descriptor (UCD) present. The DCD and UCD will be transmitted by the Base Station (BS) at a periodic interval to define the downlink and uplink physical channels. This information will be TLV encoded and may include parameters such as, the TTG and RTG times, as will be explained in more detail below, centre frequency, BS ID, frame duration and Handover type. Also contained within the DCD and UCD will be a description of the burst profiles that are used for bursts within the downlink and uplink subframes. This information will also be TLV (type/length/value) encoded and may include information such as, FEC type, encoding rate and modulation. Once defined, these profiles will then be referred to in DL and UL MAP_IE's in later frames via a numerical index called Downlink Interval Usage Code (DIUC) and Uplink Interval Usage Code (UIUC). In the IEEE 802.16 standard, different numerical values of DIUC and UIUC are used to stipulate the burst profiles being used, however some values within DIUC/UIUC can be used to denote different zone profiles such as a PAPR (Peak to Average Power Ratio) reduction zone. In this case DIUC/UIUC=13 will ensure that a PAPR reduction zone is created where the base station transmits non information carrying signals in order to reduce the peak to average ratio of the transmitted waveform, as well as providing coverage-enhancing safety zones to avoid interference with other base stations.
From decoding the DL-MAP_IE and UL-MAP_IEs (which contain the DIUC and UIUC respectively) the Mobile Station (MS) can determine the bursts and associated burst profiles (i.e the modulation and coding scheme) to which its connections are associated within the downlink and uplink subframes. If any of the configurations change within either of the TLV encoded information for the physical channel or the burst profiles then the DCD and/or UCD must be updated and transmitted as before (i.e. after the DL and UL MAPs).
Considering the case where an IEEE 802.16m BS must support some legacy MSs then the above signalling must be present in the first zone of the DL sub-frame following the preamble in order for legacy MSs to determine their resource allocations within the DL and UL sub-frames. It is anticipated however that the initial IEEE 802.16m network rollout will involve the installation of IEEE 802.16m BSs where a large percentage of terminals using these BSs will only support the legacy IEEE 802.16e-2005 standard. However, it is also anticipated that over time this large percentage will gradually decrease as most users will eventually switch from using legacy equipment to using IEEE 802.16m terminals. It would therefore be advantageous for the IEEE 802.16m frame structure to be capable of an almost seamless transition from a legacy-like system to an IEEE 802.16m system. The state of this transition will solely depend on the percentage of legacy terminals wishing to access the network. As the number of legacy users decrease then it would be expected that the performance of the IEEE 802.16m MSs should improve and vice-versa.
One major constraint in the design of an IEEE 802.16m frame structure is the preamble position that will be used for legacy synchronisation and network entry. This preamble is crucial, and in the current TDD legacy frame structure it is generated every 5 ms (according to the WiMAX forum release 1.0 profile). Therefore, this preamble must be present in the proposed frame structure which will constrain the flexibility in the frame design. As mentioned above, in order to reduce latency then the frame duration must be decreased but as a result with a TDD system this will increase the number of RTG and TTGs therefore increasing the number of wasted symbols. It is important to note that any sub-frames or zones where legacy allocations are made must begin on an integer number of symbols from the beginning of either the UL or DL legacy subframes. It is also crucial when considering the case of small sub-frame durations in the DL, that the first DL sub-frame must contain an adequate number of symbols to accommodate the cumbersome legacy signalling (i.e, FCH, DL and UL MAPs etc.).